1. Field of the Invention
The present invention concerns a system for the detection of light pulses, especially pulses emitted by a laser, with a notably low false alarm rate.
By light pulses we mean pulses with wavelengths in both the visible spectrum and the invisible spectrum, especially in the near infrared and far infrared range, for example, for laser radiation at 1.06 microns and 10.6 microns.
The false alarm rate is reduced by making the detecting system insensitive to spurious signals and, especially, to the electrical pulses created by the photodetectors themselves when there is no source of irradiating optical energy.
This result is got, according to the invention, by an arrangement of the optronic detection channels, through the parallel mounting, for each channel, of two photodetectors coupled to one and the same optical channel. Then, by correlation, a specific analog (or digital) processing of the detected pulses enables the detection and validation of the light pulses received and the rejection of the internal spurious pulses regardless of their width, amplitude and repetition characteristic.
According to an application more particularly envisaged, the invention concerns the detection of laser pulses and enables the making of equipment which can perform the function of a laser warning detector.
2. Description of the Prior Art
The frequent use of laser illuminators in weapon systems for telemetry or target designation has made it necessary to envisage efficient protection of the carriers (armored vehicles, ships or aircraft). This protection consists firstly in alerting the crew to the presence of a laser threat so as to bring about an immediate reaction: this is the basic function of the laser warning detector.
Current operational detectors emit pulses of a width equal to a few nanoseconds. A laser warning detector generally consists of several optronic sensors. Each sensor is provided with an optical part coupled to a detector giving a current which is proportionate, at all instants, to the flux that it receives. Electronic processing enables the detection of the received flux as soon as the detected signal is above a pre-determined threshold. In terms of current, this amounts to a value of the detected current greater than a minimum value IS1 resulting from the comparison threshold value used for operation.
Each optronic channel is characterized by its field which is that part of space observed by the sensor and from where the light emissions are collected, by its sensitivity which is the minimum illumination value that can be detected at the input of the sensor, and by its false alarm rate which is the number of false detections per hour of operation when there is no light energy received at all.
The field and sensitivity are thus defined by the characteristics of the illuminators, the design constraints on the equipment and the carriers to be protected. These various characteristics lead to specifying a current threshold value IS1 which should not be exceeded.
The false alarm rate characterizes the reliability of the information given by the warning detector: the required value depends on the application (the task and the carrier) and may vary between 1 and 1/1000. The value 1/1000 corresponds to one false alarm per 1000 hours of operation for all the optronic channels. False alarm rate (abbreviated as FAR) requirements often lay down a detection threshold IS2 which is greater than the above-mentioned limit value IS1 and is therefore incompatible with the field and sensitivity specifications of the equipment.
The present invention provides, through a simple arrangement of the optronic channel, for the possibility of reducing the relationship between threshold IS and the false alarm rate FAR (curve C1 of FIG. 1 corresponding to a system not arranged according to the invention) to a magnitude compatible with the limit value IS1 (the curve C2 of FIG. 2 corresponding to a system arranged according to the invention).
False alarms can be produced by electro-magnetic sources external to the equipment (for example radio transmissions, radar or electrical arcs) or internal to the equipment (such as spurious phenomena produced by certain parts of the equipment). False alarms can also result from random noise sources due to the detector and to electronic processing. Thirdly, false alarms can result from signals created spontaneously in the detecting elements. These signals take the form of very brief pulses. They are filtered by the frequency response characteristic of the detector and appear, with respect to electronic processing, in the form of waves close to the pulses produced by the laser illuminators.
The effect of the electro-magnetic sources can be reduced through appropriate design (such as shielding, filtering, and the elimination of inconvenient couplings).
The random noise sources generally remain compatible with the maximum detection threshold IS1.
On the contrary, the spontaneous generation of pulses in the detector plays a preponderant role in subsequently determining the false alarm rate.
An object of the present invention is the application of a processing method which distinguishes pulse signals of light origin or useful signals from the spurious signals and especially from those created by the detectors themselves.
The discriminating of these spurious pulse signals by waveform recognition cannot be contemplated since the waveforms are close to those of the useful signals. The time interval differences to be observed, which are of the order of a few nanoseconds, imply the use of very complicated processing. Finally, although the spectrum of the laser pulses to be considered always appears to be limited by the detector, this experimental observation cannot be made into an absolute generalization because of the various types of materials and technologies on which the detectors are based.
An aim of the invention is to remove all these drawbacks by arranging the octronic channels of the equipment in a special way.